The United States, Russia and Jordan reached the "de-escalation agreement" on Friday, following the first meeting between U.S. President Donald Trump and Russian President Vladimir Putin at the G20 summit in Germany.

The Syrian Observatory for Human Rights said on Sunday that "calm prevailed" in the southwest since the truce began, despite minor violations.

Combatants briefly exchanged fire in Deraa province and in Quneitra around midnight, but this "did not threaten the ceasefire," said Observatory Director Rami Abdulrahman.

Trump, meanwhile, welcomed the ceasefire, saying it could save many lives in the war-torn country.

"Syrian ceasefire seems to be holding. Many lives can be saved. Came out of meeting. Good!" Trump tweeted on Sunday.

A Syrian official, meanwhile, indicated that Damascus approved of the ceasefire deal, describing the government's silence over it as a "sign of satisfaction."

"We welcome any step that would cease the fire and pave the way for peaceful solutions," the government official told Reuters.

The new ceasefire deal is separate from "de-escalation zones" that were to be created under a deal brokered by Russia, Turkey and Iran earlier this year. The U.S. was not part of that deal.

A senior State Department official involved in the talks said on Sunday further discussions would be necessary to decide crucial aspects of the agreement, including who will monitor its enforcement.